Efficient generation and control of different-order orbital angular momentum states for communication links

We present an optical scheme to encode and decode 2 bits of information into different orbital angular momentum (OAM) states of a paraxial optical beam. Our device generates the four light angular momentum states of order ±2 and ±4 by spin-to-orbital angular momentum conversion in a triangular optical loop arrangement. The switching among the four OAM states is obtained by changing the polarization state of the circulating beam by two quarter-wave plates, and the 2 bit information is transferred to the beam OAM exploiting a single q plate. The polarization of the exit beam is left free for an additional 1 bit of information. The switching among the different OAM states can be as fast as a few nanoseconds, if suitable electro-optical cells are used. This may be particularly useful in communication systems based on light OAM.     

Magneto-optical properties of Tb0.81Ca0.19F2.81 and Tb0.76Sr0.24F2.76 single crystals

High optical quality terbium calcium fluoride (Tb_0.81Ca_0.19F_2.81) and terbium strontium fluoride (Tb_0.76Sr_0.24F_2.76) single crystals have been grown by the Czochralski method under controlled atmosphere. While both compounds present similar optical and magneto-optical properties (determined by the high concentration of Tb³⁺ ions), the lower absorption losses and high Faraday rotation exhibited by terbium calcium fluoride, compared to those of terbium strontium fluoride, lead to a notable figure of merit for Faraday isolators operating in the visible wavelength region. In particular at short wavelengths, below 500 nm, the figure of merit of terbium calcium fluoride is remarkably higher than that of the industrial standard terbium gallium garnet.     

Acoustooptic Properties of Cs3Bi2I9 Crystals

Cs₃Bi₂I₉ crystals were grown, and their longitudinal-wave acoustooptic properties were studied. The crystals were found to possess a large acoustooptic figure of merit and photoelastic constants.     

Effect of nonstoichiometry on the thermoelectric properties of Tl<Subscript>4</Subscript>TiS<Subscript>4</Subscript> crystals

Stoichiometric (1), Tl₂S-enriched (2), and TiS₂-enriched (3) Tl₄TiS₄ crystals (with deviations from stoichiometry within the homogeneity range of the ternary compound) have been grown by the Bridgman method, and their thermoelectric properties have been studied by the Harman technique. All of the crystals have a high thermoelectric figure of merit. The highest figure of merit is offered by crystal 1 at positive thermoelectric powers and by crystal 3 at negative thermoelectric powers. The thermoelectric properties of crystal 2 were found to irreversibly degrade over time.     

Properties of Sn-doped Bi2Te3 − x Sex single crystals

The effect of Sn doping (0.2 and 0.4 at %) on the properties of Czochralski-grown single crystals of n-type Bi₂Te_2.85Se_0.15 solid solutions is studied. Thermoelectric power, electrical conductivity, thermal conductivity, and Hall effect measurements in the range 77–400 K demonstrate that Sn doping has a significant effect on the transport properties of the solid solutions. Between 300 and 370 K, the thermoelectric figure of merit of Bi_1.996Sn_0.004Te_2.85Se_0.15 single crystals is higher than that of the Sn-free solid solution. In addition, hot-microprobe thermoelectric power measurements, highly sensitive to variations in carrier concentration, indicate that the Sn-doped single crystals are very uniform in electrical properties, both along the growth direction and radially.     

Electro‐Optic Modulation in Hybrid Metal Halide Perovskites

Rapid and efficient conversion of electrical signals to optical signals is needed in telecommunications and data network interconnection. The linear electro‐optic (EO) effect in noncentrosymmetric materials offers a pathway to such conversion. Conventional inorganic EO materials make on‐chip integration challenging, while organic nonlinear molecules suffer from thermodynamic molecular disordering that decreases the EO coefficient of the material. It has been posited that hybrid metal halide perovskites could potentially combine the advantages of inorganic materials (stable crystal orientation) with those of organic materials (solution processing). Here, layered metal halide perovskites are reported and investigated for in‐plane birefringence and linear electro‐optic response. Phenylmethylammonium lead chloride (PMA₂PbCl₄) crystals are grown that exhibit a noncentrosymmetric space group. Birefringence measurements and Raman spectroscopy confirm optical and structural anisotropy in the material. By applying an electric field on the crystal surface, the linear EO effect in PMA₂PbCl₄ is reported and its EO coefficient is determined to be 1.40 pm V^?1. This is the first demonstration of this effect in hybrid metal halide perovskites, materials that feature both highly ordered crystalline structures and solution processability. The in‐plane birefringence and electro‐optic response reveal that layered perovskite crystals could be further explored for potential applications in polarizing optics and EO modulation.     

Thermoelectric properties of <Emphasis Type="Italic">n</Emphasis>-Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript>〈I,In〉 crystals

n-Bi₂Te_2.7Se_0.3〈I〉 single crystals additionally doped with indium (0.1–2.0 mol % In₂Te₃) have been grown by Czochralski pulling through a floating crucible. As the In₂Te₃ content of the growth charge increases to 0.2 mol %, the thermoelectric figure of merit (Z) of the crystals increases from 0.0029 to 0.0031 K⁻¹ and Z _max shifts to higher temperatures by ∼30 K. Increasing the In₂Te₃ content to 2 mol % sharply reduces Z _max to 0.002 K⁻¹ and shifts the maximum to lower temperatures. This behavior of the thermoelectric figure of merit can be understood in terms of the nonmonotonic variation of electron mobility with indium and iodine concentrations in the crystals.     

Thermoelectric properties of Bi2Te3-Sb2Te3 single crystals in the range 100–700 K

The electrical conductivity, thermoelectric power, and thermal conductivity of Bi₂Te₃-Sb₂Te₃ crystals grown by the floating-crucible technique were measured in the temperature range from 100 to 700 K. The thermoelectric figure of merit of the crystals was evaluated. The effect of crystal composition on these properties was analyzed.     

Angular momentum of the fields of a few-mode waveguide: Conversion of the angular momentum

An analysis is made of the transformation of the angular momentum density in the field of an unstable IV vortex of a few-mode optical fiber. It is shown that the effect of mode dispersion of IV vortices is observed as the conversion of the polarization and orbital components of the electrodynamic angular momentum. The angular momentum defect may be recorded experimentally as a mechanical twist of the optical few-mode fiber. Formally the dispersion process resembles the conversion of the signs of the orbital and polarization components of the angular momentum density. A complex pseudopotential, whose real and imaginary parts characterize the field lines and lines of equal pseudopotential, is introduced to describe the energy flux density of the fiber vortex. The conversion of field states with equivalent partial ê ⁺ F ₁(R)exp{−iφ} and ê2⁻ F ₁(R)exp{+iφ} vortices was investigated experimentally. Pis#x2019;ma Zh. Tekh. Fiz. 23, 58–65 (November 26, 1997)     

Phase equilibria in the Tl<Subscript>2</Subscript>Se-PbSe system and growth and properties of Tl<Subscript>4</Subscript>PbSe<Subscript>3</Subscript> single crystals

The T-x phase diagram of the Tl₂Se-PbSe system has been mapped out. The system contains a congruently melting compound (803 K) of composition Tl₄PbSe₃, which forms a continuous series of solid solutions with Tl₂Se. Tl₄PbSe₃ single crystals have been grown by directional solidification, and their physicochemical and thermoelectric properties have been studied. Tl₄PbSe₃ crystals have high thermopower (α _T ) and thermoelectric figure of merit (Z _T ).     

Metallic n-Type Mg3Sb2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance

Mg₃(Sb,Bi)₂ alloys have recently been discovered as a competitive alternative to the state-of-the-art n-type Bi₂(Te,Se)₃ thermoelectric alloys. Previous theoretical studies predict that single crystals Mg₃(Sb,Bi)₂ can exhibit higher thermoelectric performance near room temperature by eliminating grain boundary resistance. However, the intrinsic Mg defect chemistry makes it challenging to grow n-type Mg₃(Sb,Bi)₂ single crystals. Here, the first thermoelectric properties of n-type Te-doped Mg₃Sb₂ single crystals, synthesized by a combination of Sb-flux method and Mg-vapor annealing, is reported. The electrical conductivity and carrier mobility of single crystals exhibit a metallic behavior with a typical T^−1.5 dependence, indicating that phonon scattering dominates the charge carrier transport. The absence of any evidence of ionized impurity scattering in Te-doped Mg₃Sb₂ single crystals proves that the thermally activated mobility previously observed in polycrystalline materials is caused by grain boundary resistance. Eliminating this grain boundary resistance in the single crystals results in a large enhancement of the weighted mobility and figure of merit zT by more than 100% near room temperature. This work experimentally demonstrates the accurate understanding of charge-carrier scattering is crucial for developing high-performance thermoelectric materials and indicates that single-crystalline Mg₃(Sb,Bi)₂ solid solutions can exhibit higher zT compared to polycrystalline samples.     

Ultra-high average figure of merit in synergistic band engineered SnxNa1−xSe0.9S0.1 single crystals

Thermal-electricity conversion is one of the most promising routes to harvest heat and convert it as easily storable and deliverable electric energy. Signi?cant progress has been made since the discovery of Seebeck effect in 1821, particularly, the figure of merit zT approached a record high value of 2.6 in 2014. However, for thermoelectric devices, high average zT values (zT_ave) over the operating temperature range is more important as it is directly related to the conversion efficiency ($\eta$). Approaching highly stable and repeatable ultra-high zT_ave for Te-free materials has been historically challenging over the past century though exciting progress with zT_ave well above 1.10 was made recently. Here, through synergistic band engineering strategy for single crystalline SnSe, we report a series of record high zT_ave over a wide temperature range, approaching ～1.60 in the range from 300?K to 923?K in Na-doped SnSe_0.9S_0.1 solid solution single crystals, with the maximum zT of 2.3 at 773?K. These ultra-high thermoelectric performance derive from the new multiple valence band extrema near the band edges in SnSe_0.9S_0.1 and the shift of Fermi level towards the multi-valley bands through Na doping which introduce additional carrier pockets to attend electrical transport. These effects result in an optimized ultrahigh power factor exceeding 4.0?mW?m^?1?K^?2 in Sn_0.97Na_0.03Se_0.9S_0.1 single crystals. Combined with the extremely lowered thermal conductivity attributed from the intrinsic anharmonicity and point defect phonon scattering, the series of ultra-high zT_ave and a record high calculated conversion efficiency of 21% over a wide temperature range are approached.     

Commutation Rules and Eigenvalues of Spin and Orbital Angular Momentum of Radiation Fields

Abstract

We investigate the separation of the total angular momentum J of the electromagnetic field into a ‘spin’ part S and an ‘orbital’ part L. We show that both ‘spin’ and ‘orbital’ angular momentum are observables. However, the transversality of the radiation field affects the commutation relations for the associated quantum operators. This implies that neither S nor L are angular momentum operators. Moreover their eigenvalues are not discrete. We construct field modes such that each mode excitation (photon) is in a simultaneous eigenstate of S _z and L _z. We consider the interaction of such a photon with an atom and the resulting effect on the internal and external part of the atomic angular momentum.     

Acousto-optic properties of LiBi(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystals grown at low temperature gradients

This paper reports the starting charge composition and process parameters for low thermal gradient (<1°C/cm) Czochralski growth of uniform bulk LiBi(MoO₄)₂ crystals. The acousto-optic figure of merit (M ₂) of a LiBi(MoO₄)₂ crystal has been measured in different directions.     

Physical properties of Bi2Te2.85Se0.15 single crystals doped with Cu, Cd, In, Ge, S, or Se

Bi₂Te_2.85Se_0.15 crystals doped with Cu, Cd, In, Ge, S, or Se were grown by the floating-crucible technique. The effective segregation coefficients for the dopants were determined. The thermoelectric power, electrical conductivity, and thermal conductivity of the samples were measured in the temperature range from 77 to 350 K. The effects of the dopants studied on the temperature dependences of the electrical properties and thermoelectric figure of merit were examined. The bending strength of the doped crystals was measured.     

High performance functionally graded and segmented Bi2Te3-based materials for thermoelectric power generation

Bi₂Te₃-based materials possess a figure of merit maximum over a narrow temperature range. When used in a generating mode over a large temperature difference the material operates at a substantially lower overall figure of merit than its maximum value. The conversion efficiency of a thermoelectric generator for low temperature waste heat recovery can be increased by employing functionally graded or segmented materials. In this work functionally graded p-type Bi₂Te₃-based thermoelectric materials have been prepared from melt by the Bridgman method using double doping technique. Segmented n-type thermoelement has been fabricated by joining two Bi₂Te₃-based materials with figure of merit maximum at 270 K and 380 K. The thermoelectric properties of the materials and a thermocouple comprised of p-type functionally graded and n-type segmented materials have been measured over a temperature range 200 K–450 K. The material efficiency of the thermocouple over the temperature gradient 223 K–423 K is estimated to be 10% compared with 8.8% for a standard Bi₂Te₃-based materials.     

Angular momentum of the fields of a few-mode fiber. III. Optical Magnus effect, Berry phase, and topological birefringence

It is shown that, on the one hand, the evolution of the angular rotation of the lines of nodes of the CP₁₁ mode is a manifestation of the optical Magnus effect in a few-mode fiber with a parabolic refractive index profile, and, on the other hand, the additional phase γ _b =±δβ ₂₁ z in CV and IV vortices is the Berry topological phase, which arises as a result of the cyclic change in the orientations of the orthogonal axes of dislocations. The splitting of the propagation velocities of orthogonal circularly polarized CV⁺ and IV⁻ modes in an LV vortex in a parabolic fiber is a manifestation of the phenomenon of topological birefringence of a few-mode fiber. The azimuth of the linear polarization of a vortex undergoes continuous angular rotation. In an optical fiber with a stepped index profile the CP₁₁ mode forms circularly polarized edge dislocation over lengths which are multiples of half the beat length, and over lengths which are odd multiples of the quarter beat length it forms linearly polarized fields with a purely screw dislocation. This transformation of edge and screw dislocations can be regarded formally as conversion of the polarizational angular momentum into orbital angular momentum. The conversion of angular momentum is a reflection of the dynamical unity of the optical Magnus effect and the Berry topological phase in the fields of a few-mode fiber. Pis’ma Zh. Tekh. Fiz. 23, 59–67 (December 12, 1997)     

Spin-to-orbit conversion at acousto-optic diffraction of light: conservation of optical angular momentum

Acousto-optic diffraction of light in optically active cubic crystals is analyzed from the viewpoint of conservation of optical angular momentum. It is shown that the availability of angular momentum in the diffracted optical beam can be necessarily inferred from the requirements of angular momentum conservation law. As follows from our analysis, a circularly polarized diffracted wave should bear an orbital angular momentum. The efficiency of the spin-to-orbit momentum conversion is governed by the efficiency of acousto-optic diffraction.     

Effects of growth-charge purity and perfection of Bi0.5Sb1.5Te3 single crystals on their thermoelectric properties

Bi_0.5Sb_1.5Te₃ crystals, undoped and doped with 4 mol % Bi₂Se₃, were grown by the floating-crucible technique, using starting materials of 99.9999 or 99.99% purity. The perfection of the crystals (mosaic blocks, second-phase inclusions, dislocation density) was correlated with growth conditions. The elemental composition of several samples was determined by electron probe x-ray microanalysis. The room-temperature electrical conductivity, thermal conductivity, thermoelectric power, carrier concentration, and effective mass and mobility of holes were determined. In the range from 100 to 400 K, the electrical conductivity, thermoelectric power, thermal conductivity, and thermoelectric figure of merit of the crystals were measured.     

Anisotropy of acousto–optic figure of merit in lithium tetraborate crystals

We analyse anisotropy of acousto–optic figure of merit (AOFM) for Li₂B₄O₇ crystals in order to estimate the prospects of these crystals in acousto–optics. We find that the maximal AOFM, 3.44 × 10^?15 s³/kg, is peculiar for the isotropic acousto–optic interaction of the incident ordinary optical wave with the quasi-longitudinal acoustic wave. For the case of anisotropic diffraction in Li₂B₄O₇, the maximum 1.87 × 10^?15 s³/kg can be reached using the interaction of the extraordinary optical wave with the quasi-longitudinal acoustic wave. The case of collinear diffraction is characterized by small AOFMs, with the largest value 0.26 × 10^?15 s³/kg.